An object of the invention is a radiation detector, notably a radiation detector wherein the sensitive element is made of superconductive material.
More particularly, the invention concerns configurations of sets of sensitive elements made of superconductive materials with a grain structure by which it is possible to obtain images in incoherent radiation, with a wavelength in the range between the visible wavelengths and the dwarf wavelengths.
The superconductive material may have a high critical temperature and may be used notably in a temperature range extending from 77 K (liquid nitrogen) to 27 K (liquid neon).
A planar element made of a granular superconductor 1, as shown in FIG. 1, having a current I flowing through it and being illuminated by a radiation spot with a wavelength between a visible wavelength and a dwarf wavelength undergoes local change in its resistance.
The interaction between the superconductor and the incident radiation takes place according to two physical processes depending on whether the wavelength is smaller than about some tens of micrometers or whether the wavelength is greater than some tens of micrometers.
If the wavelength of the radiation illuminating the superconductive material 1 of FIG. 1 is smaller than some tens of micrometers, then the energy of the photons is sufficient to break the Cooper pairs into quasi-particles that spread in the grains and get recombined. Two types of phenomena can be observed:
a) The populations of Cooper pairs and quasi-particles are in a state of equilibrium: the radiation heats the superconductive element (bolometrical effect represented by the zone "a" of the curve .DELTA.V as a function of the temperature of FIG. 2). The width of the forbidden band of the superconductor diminishes when the temperature increases.
b) The populations of pairs and of quasi-particles are in a state of disequilibrium. The width of the forbidden band is reduced by a value proportionate to the excess of quasi-particles (zone "b" of the curve .DELTA.V as a function of the temperature of FIG. 2).
In a granular superconductor, the grain boundaries constitute barriers of Josephson junctions. When the width of the forbidden band is reduced, the density of the critical current of the grain boundaries is reduced and becomes smaller than the density of current given. Under these conditions, the magnetic field penetrates the vortex-shaped grain spaces which move under the effect of the Lorentz force in dissipating power. A radiation thus induces a photoresistance in the illuminated zone and the amplitude of the response is proportionate to the number of moving vortices. FIG. 2 shows, by way of an example, the response as a function of the temperature. The types of response "a" and "b" are clearly distinguished.
If the wavelength of the radiation illuminating the superconductive material 1 is greater than a few tens of micrometers, the energy of the photons is smaller than the width of the forbidden band. However, the currents compensating for the magnetic field are greater than the critical currents of the grain boundaries and the magnetic field penetrates the boundaries in the form of vortices that move in dissipating power.
The invention applies this phenomenon of induction of a photoresistivity in a superconductive material, under the effect of an illumination by radiation.
An elementary detector such as this has the advantage, as compared with known photoconductor detectors, of enabling the making of sensitive detection elements having smaller dimensions. For, in the devices according to the invention, the sensitive part is restricted to the part illuminated by the radiation beam to be detected whereas, in photoconductors, there is a spreading of the sensitive zone as compared with the illuminated zone as shall be seen further below.
Furthermore, certain embodiments of the invention enable operation without electrical dissipation in darkness. This is not the case with photoconductive devices.
The device according to the invention makes it possible to obtain detectors with large areas and enables operation in a wide range of wavelengths.